(1) Field of the Invention
The invention relates to power supplies in general and more particularly to switch mode power supplies in which a power transformer is directly coupled to the bulk voltage.
(2) Prior Art
The use of switch mode power supplies for providing power to different types of loads is well known in the prior art. Generally, the prior art switch mode power supplies consist of a power transformer with an input coil or winding and an output winding. The input winding is connected to a supply voltage which provides electrical energy to the transformer. A switching circuit, which may be a bridge or a two-switch push pull design, is connected to the input winding. The circuit switches the direction of magnetic flux within the input winding as a result of voltage developed across the input winding. The voltage is induced onto the output winding. The voltage is then rectified and supplied to the attached load. A feedback error voltage is developed and is used to generate pulse width modulated signals which drive the switching circuits.
Such prior art power supplies, especially the double-ended, direct-coupled type, are notorious for becoming unstable and ultimately failing. The defect is even more pronounced when they are used with highly active loads. It is believed that the problem arises when the transformer is saturated. The saturation occurs when the magnetic flux density B, and/or the coercive force H exceeds safe bounds for the magnetic material used to manufacture the transformer, and the magnetizing inductance I.sub.m is reduced from its nominal value.
The prior art uses several methods for solving the saturation problem. Some of these methods will now be described.
One of the prior art techniques involves sensing the current in each of the two switches directly and using the signal to control the switch turnoff signals. This technique is referred to as a current balancing or current mode technique. The current has a ramp or saw tooth waveform and it is compared to a reference voltage. The intention is to keep the switch currents equal and indirectly keep the DC component of the magnetizing current (I.sub.m) at 0. This technique presents several problems. Essentially, bipolar switches usually have unpredictable turnoff times which may be a substantial portion of on-time, so the true peak switch current, and the current expected and measured by the control system are quite different from one another. The result is that a DC component is added to I.sub.m, tending to cause saturation.
The necessary response to the saturation problem is to use a gapped-core transformer that can tolerate considerable turn-current (NI) product offset, thereby necessitating larger, more expensive magnetic and switch components and decreased efficiency. Secondly, variations in load current, even though it may be filtered, will be reflected to the primary. The reflected current is indistinguishable from changes in magnetizing current. As the system attempts to equalize switch currents while there are changes due to load conditions, the balancing technique will cause some offset in I.sub.m, and in turn result in a saturation problem. Variations in control voltage (V.sub.c), typically due to feedback and generally of a large magnitude, cause variations in switch current even if this necessitates a further NI offset. All these problems result in the designer's selecting an oversized gapped-core transformer, oversize switches, and careful filter design. An example of this technique is described in an article entitled, "Analysis of the Static Characteristics and Dynamic Response of Push-Pull Switching Converters Operating in the Current Programmed Mode," by Andersen, B. E. et al and published in Proceedings, Power Electronics Specialist Conference, 1981, pp. 29-38, IEEE.
In another technique an inductor is placed in series with the DC input terminal of the transformer primary (center tapped only). This tends to hold the primary current constant, which tends to maintain transformer stability. Non-dissipative coupling of the inductor's stored energy is a problem. Also, leakage in coupled inductors causes spikes which appear on the switches. Leakage also causes a problem with regulation and efficiency, especially in systems with power over about 200 watts. Snubbers interfere with efficiency, ripple and regulation. A practical coupled inductor has to meet isolation specifications which cause increased leakage inductance, and the inductor is expensive. Reflected load currents can cause an I.sub.m offset. Even in lieu of load-related problems, there will be some offset current due to component tolerances, and the transformer will operate at one end of the B-H loop. The effect of the inductor, in limiting saturation current, is to increase output voltage ripple at the switching frequency, which is attenuated approximately 12 db less than normal ripple components. A more detailed discussion of this technique is given in an article entitled, "Push-Pull Current-Fed Multiple-Output Regulated Wide-Input-Range DC/DC Power Converter with Only One Inductor and with 0 to 100% Switch Duty Ratio: Operation at Duty Ratio Below 50%," by Redl, R. and Sokal, N. 0., and published in Power Electronics Specialist Conference, 1981, pp. 204-212, IEEE.
Another method is the flux-analog controlled method. This method is described by Wilson, Dick in an article entitled, "A New Pulsewidth Modulation Method Inherently Maintains Output Transformer Flux Balance," Proceedings of Powercon 8, pp. D-1, 1-D1, 15, Power Concepts, Inc. In this approach an analog of the flux is used to control switch timing, with the premise that if the flux change in each direction is exactly balanced the transformer will not saturate. The flux analog is derived by integrating the voltage on an auxiliary winding of the power transformer. One problem with this technique is that a practical integrator is imperfect at DC and low frequencies, and there is an additive constant error by virtue of the indefinite integral. To compensate, a circuit is added to detect the difference in peak switch currents, and this result is applied to the integrator in such a way as to modify switch times to equalize the peak currents. This technique is effective except for the aforementioned problem of reflected synchronous or transient load current at the primary. As previously stated, the reflection saturates the transformer. It is therefore necessary to ensure that the magnetizing current is much greater than the maximum reflected AC load current at the switch frequency, necessitating a gapped-core transformer or special output filter.
In yet another prior art method the switches driving the primary are essentially controlled in a standard pulsewidth modulated (PWM) fashion. The method senses saturation at the core directly, via an evenly-gapped E-E core and a special winding. The information, representative of core saturation, is used to modify switching times to correct the saturation problem. The gapped transformer implies problems with size, weight, cost and efficiency. In particular, the specially made transformers are expensive and of limited application. The introduction of an air gap implies large magnetizing current. A more detailed description of this technique is given by Patel, Raoji, in an article entitled, "Detecting Impending Core Saturation in Switched-Mode Power Converters," Unitrode Power Supply Design Seminar, 1980, Unitrode Corporation, Lexington, Mass.
As is obvious from the above description, all previous solutions utilize either a special, complicated transformer to find the effects of saturation, or they base the protection method on measurement of the primary current, I.sub.p. Since in the desired mode of operation the magnetizing current I.sub.m is much smaller than I.sub.p, the prior art technique does not give an estimate or measure I.sub.m. It merely defines I.sub.m as having already become excessively high.